1. Field of the Invention
This invention relates to a stain determination apparatus that determines stain degrees of printed matters such as sheets, a stain determination method and a sheet processing apparatus having the stain determination apparatus.
2. Description of the Related Art
Conventionally, as a method for determining the stain degree of a printed matter, a method for determining the stain degree in an image of a printed matter read by use of a camera or scanner is proposed. For example, there is provided a method for determining the stain degree based on an average value or dispersion value of concentration values in an image read from the printed matter. Further, as a method for determining a variation between a plurality of images, a method using an image analysis process by treating values of red (R), green (G) and blue (B) configuring a color image as 3-dimensional vectors is provided (for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-117400. In this case, it is possible to determine a variation between the images according to distances between the 3-dimensional vectors configured by the values of the three colors of R, G and B. By applying the above method, a method for determining the stain degree of an image read from the printed matter may be considered.
However, it cannot be avoided that a certain variation occurs in the printed matter in the manufacturing process such as a printing process. That is, even in printed matters on which the same patter is printed, variations (that are hereinafter referred to as inherent variation components) caused by factors other than stains may be contained in the printed matters. This means that the concentration average values calculated based on image information items of a plurality of printed matters do not completely coincide even if the same pattern is printed on the printed matters without causing any stain. That is, the variation component inherent to the printed matter acts as a factor that gives a bad influence on the stain degree determination precision.